Ongoing efforts are being made in the field of microelectronic devices to achieve a higher circuit density. One method of increasing the number of components per chip is to decrease the minimum feature size on the chip, which requires higher lithographic resolution. This was accomplished over the years by reducing the wavelength of the imaging radiation from the visible (436 nm) down through the ultraviolet (365 nm) to the deep ultraviolet (DUV) at 248 nm. Development of commercial lithographic processes using ultra-deep ultraviolet radiation, particularly 193 nm and 157 nm, has become of significant interest. See, with respect to 193 nm resists, Allen et al. (1995), “Resolution and Etch Resistance of a Family of 193 nm Positive Resists,” J. Photopolym. Sci. and Tech. 8(4):623, and Abe et al. (1995), “Study of ArF Resist Material in Terms of Transparency and Dry Etch Resistance,” J. Photopolym. Sci. and Tech 8(4):637.
In order for photoresists to function properly, their films must be transparent enough at the exposing wavelength to enable sufficient light to penetrate to the bottom of the film to create usable developed relief images. This generally corresponds to a maximum absorbance of up to approximately 0.4 or 0.5 for the required film thickness. The poor transparency at 157 nm of the polymers currently used in 248 nm (primarily p-hydroxystyrene based) and 193 nm resists (polymethacrylates and norbornene-maleic anhydride co- and terpolymers) is well known and there is some level of understanding of what types of polymers are transparent at 157 nm. See Kunz et al. (1999), “Outlook for 157 nm Resist Design,” Proc. SPIE 3678, 13. The most transparent materials identified to date, heavily fluorinated polymers such as polytetrafluoroethylene (e.g., Teflon AF®; see Endert et al. (1999) Proc. SPIE-Int. Soc. Opt. Eng, 3618:413) or hydridosilsesquioxanes (see U.S. Pat. No. 6,087,064 to Lin et al.), are not suitable because they do not have the requisite reactivity or solubility characteristics. The challenge in developing chemically amplified resists for 157 nm lithography is in achieving suitable transparency in polymers that can be developed efficiently using industry standard developers.
Homo- and copolymers of methyl α-trifluoromethylacrylate (MTFMA) have been found to be surprisingly transparent at 157 nm, exhibiting an optical density (OD) of 3/μm, while poly(methyl methacrylate) (PMMA) is highly absorbing (exhibiting an OD of 5.7/μm at 157 nm). Unfortunately, however, MTFMA is reluctant to undergo radical homopolymerization and homopolymer can be made only by anionic polymerization. Its incorporation into copolymers with methacrylates is significantly less than 50%. See Ito et al. (1981), “Methyl Alpha-Trifluoromethylacrylate, an E-Beam and UV Resist,” IBM Technical Disclosure Bulletin 24(4):991; Ito et al. (1982), “Polymerization of Methyl α-(Trifluoromethyl)acrylate and α-(Trifluoromethyl)-acrylonitrile and Copolymerization of These Monomers with Methyl Methacrylate,” Macromolecules 15:915; Willson et al. (1983), “Poly(methyl α-Trifluoromethylacrylate) as a Positive Electron Beam Resist,” Polymer Engineering and Science 23(18):1000-1003; Ito et al. (1984) “Radical Reactivity and Q-e Values of Methyl α-(Trifluoromethyl)acrylate,” Macromolecules 17:2204; and Ito et al. (1987), “Anionic Polymerization of α-(Trifluoromethyl)Acrylate,” in Recent Advances in Anionic Polymerization, T. E. Hogen-Esch and J. Smid, Eds. (Elsevier Science Publishing Co., Inc.).
Certain norbornene derivatives have been identified as comonomers that undergo radical copolymerization with α-trifluoromethylacrylic monomers, as described in U.S. Pat. No. 6,509,134 to Ito et al. and in U.S. Patent Application Publication No. 2002/0102490 A1. In addition, it has been demonstrated, quite recently, that MTFMA and other α-trifluoromethylacrylic esters undergo radical copolymerization with various vinyl ether derivatives, which has opened up more possibilities in the design of 157 nm and 193 nm bilayer and single layer resist materials; see, e.g., U.S. patent application Ser. No. 10/091,373 to Ito, filed Mar. 4, 2002, for “Copolymer for Use in Amplification Resists,” assigned to International Business Machines Corporation. Vinyl ethers have been copolymerized with maleic anhydride (MA) for the design of 193 nm resists, as described by Choi et al. (2000), “Design and Synthesis of New Photoresist Materials for ArF Lithography,” Proc. SPIE 3999:54. A third functional monomer, however, had to be terpolymerized with the vinyl ether and MA because neither the vinyl ether used nor the MA was functionalized. In addition, incorporation of a conventional vinyl ether into a copolymer does not increase the polymer's polarity, and so does not enhance the efficiency of resist development in industry standard developers. Furthermore, conventional vinyl ethers do not enhance the 157 nm transparency of a copolymer containing an α-trifluoromethylacrylate co-monomer.
There is, accordingly, a need in the art for new polymers that exhibit enhanced transparency at 157 nm and contain a sufficient number of polar groups so that solubility in industry standard developers, particularly aqueous base, is improved relative to the solubility of previously disclosed polymers used in 157 nm resists.